National Express coaches are laying on 10,000 extra seats over the holiday weekend. Photograph: Newscast/Alamy

Motoring organisations have warned that this weekend could see some of the heaviest spring bank holiday traffic in years, with 17m cars on the road as families also take advantage of the half-term holiday. The AA and Trafficmaster said traffic was likely to be exceptionally busy on main trunk routes from afternoon on Friday and during Saturday.

Rail passengers will face disruption on some routes over a weekend that weather forecasters say will bring a mixed bag of sunshine and showers.

The AA said the busiest roads were likely to be the M25 between the M3 and M4 and around the Dartford river crossing; the M1 north of the M25 to Milton Keynes and between junctions 25 and 31 (M18); the M6 both ways through the West Midlands; the M5 southbound; and the A303 and the M4 westbound from west London to Newbury, as well as stretches into Wales.

The Highways Agency said it would complete 180 miles of roadworks on England's motorway and A-road network by tomorrow, and would suspend a further 145 miles of works.

On the railways, buses will replace trains on a number of routes, although the Rail Delivery Group said there would be more services running than this time last year, with almost 8% fewer rail replacements needed. Work on the West Coast mainline will mean reduced services from London to Birmingham and Manchester from Saturday evening until Monday afternoon.

National Express is laying on 10,000 extra seats on coaches over the weekend, with Saturday expected to be the company's busiest day.

Gatwick is expecting 270,000 passengers to fly out over the weekend, and 628,000 over half-term week, with Barcelona, Malaga and Palma the most popular destinations.

The Heathrow Express rail link is hoping to run a normal service despite disruption from a strike by the RMT. A potential strike by power staff on the London Underground is not expected to affect passengers.